This invention relates to an improved system and method for regenerating spent activated carbon. It is useful for the regeneration of carbon exhausted, for example, at food processing plants, water pollution control facilities, wastewater treatment plants and industrial wastewater purification systems.
The regeneration of spent carbon requires drying the carbon at a temperature up to 100.degree. C., followed by baking up to, say, 600.degree. C. to volatilize the adsorbates, followed by activating the carbon at a temperature up to, say, 800.degree. C. in the presence of steam to oxidize the carbon surfaces freed from the decomposed adsorbates.
Prior art systems and methods for thermal regeneration comprise introducing spent activated carbon into a furnace chamber which is more or less indirectly heated while introducing steam. Typically, the indirect heating involves combustion of gases as, for example, is taught in U.S. Pat. Nos. 2,966,447; 4,007,014; and 4,008,994. One prior art patent suggests use of infrared heating lamps to heat the carbon material passed thereunder on a moving conveyor. See U.S. Pat. No. 4,050,900.
An electrically heated carbon regeneration process has great appeal because it can enable economic recovery at small capacities. Until this time, due to the capital expense of gas-fired rotary hearth-type furnaces, small capacity, say 100 to 200 pounds per hour, systems were not considered economic. Moreover, whenever gas-fired furnaces are used it is necessary to have well-trained operators pay very close attention to the operation of the furnace. Electric heating is much less dangerous and requires less tending. While electric heating is much more efficient than gas heating, with the greater cost per BTU for electric energy the operating costs (for energy) are about the same for both gas and electric heating. For the small capacity operation where spent activated carbon materials is now being discarded rather than reactivated, the electrically heated system described herein fulfills a definite need.
Today, most activated carbon is regenerated in gas- or oil-fired multiple hearth furnaces or rotary kilns. Dewatering of the carbon is performed in a dewatering screw which only reduces the moisture content to about 50% by weight. The result is that a large amount of energy is used to vaporize and heat the moisture along with the carbon. Thus a typical gas heated furnace presently uses approximately one million BTUs of energy from natural gas in order to regenerate 166 pounds per hour of carbon. This corresponds to approximately 6000 BTUs per pound. In an electric furnace with a solid-liquid separator as described in this application, the same quantity of carbon would be regenerated using 119,000 BTUs of electric energy per hour. This corresponds to approximately 700 BTUs per pound. This saving in total energy requirement is, of course, substantial.
Typically about 150 pounds per hour steam is required with the gas-fired furnace. With the electric-heated furnace described herein, steam generation is no longer required.
When a gas-fired furnace starts from a cold condition, the refractory lining of the furnace is warmed for about two days before regeneration begins. The electric furnace described herein is insulated with material which may have an insulating value of approximately three times the value of refractory linings in gas-fired furnaces. Therefore, the electric furnace will be able to output regenerated carbon approximately one-half hour after the decision is made to begin regeneration. Heat now expended to warm the furnace is therefore saved.
With the present equipment it is quite difficult to perform an optimum regeneration job. There are air leaks in the furnace inherently associated with the burners. With the furnace according to this invention, both the top, bottom and doors are completely sealed and therefore the furnace atmosphere can be controlled more exactly and the excess combustion of good carbon can be avoided. Carbon losses are reduced. Losses due to the mechanical grinding of carbon will also be reduced as the dewatering screw is not used and a rabble with teeth that rub against the carbon is not necessary. In the furnace described herein, the carbon is gently conveyed by gravity downward through the furnace.
Existing gas-fired furnaces exhaust approximately 16,000 scfh of exhaust gases at a regeneration rate of 166 pounds of carbon per hour. The electric regeneration furnace exhausts less than 1/20th of this amount.